Manufacture of electric blasting caps



Jan.

6, 1959 -J. J. O'NEILL, JR 2,867,147

MANUFACTURE OF ELECTRIC BLASIING CAPS Filed Sept. 27, 1954 FIG.

FIG. 2

I I 4 4,; j/: 4/@ I H FIG.4- INVENTOR.

JOHN J. O'NEILL JR.

AGENT MANUFACTURE OF ELECTRIC BLASTING CAPS John Joseph ONeill, Jan, Alton, IlL, assignor to Olin Mathieson Chemical Corporation, East Alton, IlL, a corporation of Virginia Application September 27, 1954, Serial No. 458,442

3 Claims. (Cl. 86--1) This invention relates to electric blasting caps and more particularly to explosive devices housed in thermoplastic shells.

Explosive devices such as plastic blasting caps have been proposed heretofore; but considerable difiiculty has been encountered in obtaining a waterproof seal between the blasting cap case or shell and the bridge plug.

It is therefore an object of this invention to provide a waterproof plastic detonator overcoming the disadvantages of previous thermoplastic detonators. Another object of this invention is to provide a novel thermoplastic detonator. A more specific object of this invention is to provide a novel solvent-less method of sealing thermoplastic detonators. A further object is to provide a method of sealing detonator components formed of difiicultly soluble thermoplastic materials. A still further object, is to heat seal a thermoplastic explosive device in a safe and eficient manner.

The manner in which these and other objects are ac-- complished in accordance with this invention will be fully understood when the following description thereof is'read in connection with the drawing, in which:

Figure 1 is a plan view of a thermoplastic detonator in posit-ion to be sealed in accordance with one embodiment of this invention,

, Figure 2 is a cross-sectional view illustrating a detonator sealed in accordance with an embodiment of this invention,

Figure 3 is a plan view of one type of heat-sealing tool that may be used in carrying out the process of this invention, and

Figure 4 is a cross sectional view taken near the tip of the tool shown in Figure 1.

In accordance with this invention, the foregoing objects are accomplished, generally speaking, by applying heat to the thermoplastic material at the juncture between the thermoplastic case housing the charge of explosive and the thermoplastic closure for the case so as to cause interfiow of the plastic materials adjacent the juncture. In other words, this invention contemplates a method of sealing explosive devices such as plastic electric blasting caps in which the external juncture of the bridge plug and case is heated at least to the melting points of the thermoplastic materials composing the bridge plug and case, and pressure is applied in the vicinity of the juncture in such a manner as to induce the plastic materials making up the component parts to flow together, thus forming a waterprof seal. More specifically, the objects of this invention may be attained by forming two substantially parallel and adjacent furrows, one on either side of the external juncture, by means of a heated member, whereby the material displaced by the formation of the furrows intermixes and forms a ridge over the external line of contact between the bridge plug and shell wall. As a result of this process, the external line of contact between the bridge plug and the detonator case is covered by a beading formed by a combination of the materials displaced from the plug and case.

tnt

Alternatively, a single furrow can be formed along the line of juncture of the plug and the case, equal quantities of thermoplastic material being displaced from each member, and then filled with the displaced thermoplastic material while still in a molten condition, so as to form a uniform seal over the original juncture. When this variation of the invention is utilized, the surface over the original juncture is smooth and unbroken, and the original configuration of the explosive device is maintained.

The temperature to which the materials are heated in accordance with the process of this invention is not critical, but in general it must be in a range above the melting points of the plastic materials being treated and usually must be below their decomposition points. Temperatures above the decomposition points may be tolerated, however, in instances when the duration of the heating is sufiiciently brief to prevent significant deterioration of the plastic material.

The seal between the bridge plug and the case is formed by the application of a grooved heated member to the detonator, the groove spanning the external joint between the bridge plug and the case. The grooved tool may be rotated about the detonator, or if desired the tool may be maintained stationary while the detonator in contact with it is rotated so that the seam between the bridge plug and the detonator case is effectively welded, in either event contact between the heated tool and the full length of the juncture line being achieved by relative rotation of the heated member and the detonator. Alternatively, the heating member may be of a type that completely encircles and embraces the detonator at the point of juncture between the bridge plug and detonator housing, and when such a tool is utilized relative rotarymotion between the detonator and the tool is not essential to the formation of a waterproof seal.

While it is preferable to heat and groove the detonator in the same operation, this invention also encompasses processes in which that portion of the detonator being treated is heated as by the application of a heating tool, and while thus heated and in a readily deformable state is brought into contact with an unheated forming tool to complete the operation.

The process of this invention is particularly well adapted to provide an efi'lcient means for sealing electric detonators having a thermoplastic case which is closed by means of a two-diameter or shouldered thermoplastic bridge plug as shown, for example, in U. S. 2,382,921, issued August 14, 1945, to Seavey. When the invention is utilized in sealing this type of detonator, the sealing operation is, of course, executed on the lateral surface of the detonator. It should be understood however that this process is equally applicable to the sealing of electric detonators in which the top surface of the bridge plug is flush with the top edge of the detonator case. When used in connection with this type of cap, the heating member is of course contacted with the annular juncture between the plug and the case, and the plug is maintained in position by means of internal stops or by the internal taper of the shell wall.

The term thermoplastic as used herein denotes resinous materials that are rendered soft, moldable and capable of fusion by the action of heat. Any thermoplastic suitable for the formation of the case may be used, examples being synthetic resins such as, for instance, cellulose esters, cellulose ethers, polyvinyl chloride, polyvinyl acetate, polyethylene, acrylic and methacrylic polymers, polystyrene, and polystyrene co-polymers, polyamides, ethyl cellulose, chlorinated rubber, and modified isomerized rubber, as well as natural resins such as, for instance, copols, accroides, and protein plastics and combinations thereof, including where desirable suitable plasticizers.

deem-4.7

the term explosive device is used to-deno-te; all, types of explosive initiators contained in a plastic housing,

The various features of the invention, will become-- more apparent and will be readily understood by those: skilled in the art when the illustrative embodiment of the:

invention is read, in connection with the attached draw-- ing. c

As shown in Figure 1, a detonator loaded with an appropriate explosive train enclosed in a polyvinyl chloride case 1, the mouth of which is closedby a bridge plug 2 of the same compositiomissecured ina chuck 4 capable of being rotated. The bridge plug is provided with a pair of lead wires which may be joined together after emergence from the plug as shown at 3.. The detonator is then rotated axially at about 80 revolutions per minute and'tool 5 which is maintained ata temperatureof about.

145 C. is brought into contact with the rotating detonator, insuch a manner that groove 6 in the face thereof spans; the lineof juncture between the detonator case 1 and the bridge plug 2, and the axis of the groove is parallel to-this line of juncture. After the tool has been brought into contact with the detonator, it is moved in slightly to make the plastic material flow toward the middle of the groove, where it is ironed smooth. In the present example, wherein the external. diameter of the detonator case is about 0.275 inch and the shell wall is about .037 inch thick, it was found that best results were obtained when the heating tool was moved about & inch toward the center of the detonator after contact was made with its periphery while the toolwasmoved about inch tangentially.

,It will be-understood, of course, that the amountcof. deformation wil. not necessarily be the same in all in-- stances, but that it will vary with the sizeof the detonatorand the characteristics of the materials being joined together. The deforming contact must be maintained until the materials formingthe bridge plug and the shell are substantially intermixedzto form a uniform beading covering the original gap between the bridge plug and the detonator housing, thus providing an effective seal. In the present case, this result was obtained by maintainingv such contact-for about 8 seconds using a single tool to deform, mix and form the seal ring.

As shown in the sealed detonator of Figure 2, the bridge plug and case are fused together at their point of contact and the original boundary line 7 has. been eliminated throughout a substantial portion of the wall thickness, being replaced bybeading 8 which is a welded mix? ture of the bridge plug and shell components; While heat sealing of a detonator charged with explosive would ordinarily be considered a hazardousun'dertakingby those skilled in the art, it will be: seen that such heat scaling is accomplished in accordance with this invention by a safe: and eflieient method lending itself to mass-production.

'For purposes ,of illustration in the present example, thedetonator lead wires were-severed "a few= inches firom. the bridge plug; but in: practice, lead wires ofnbrmal. length can be rotated along with the detonator-by conventional means not forming a part; of thepresent. invention.

Although the above specific embodiment is concerned with an. electric blasting cap in which. the bridge plug and'detonat-or case are both formed of the same material, viz., polyvinyl chloride, it will be: readily appreciated that the bridge plug and case can be-formed ofi dissimilar materials, provided they are joinable. by fusion.

Figure 3 illustrates another type of heat-sealing instrw plastic'materials of which the bridge plug and detonator 1 rnent that may be employed to obtain the benefits of this invention. This tool consists essentially of two hinged members 7 having cooperating semi-circular depressions 8 adapted to embrace and encircle an electric detonator when members 7 are brought into contact with one ant other. When this tool is used it is heated by appropriate means to a point above the melting points of the thermocase are composed. After a plastic detonator has been positioned within the'sem-i-circul-a-r depress-ions 7 in such a manner that the line of juncture between the bridge plug and the detonator casing is intermediate the parallel forming lands 9, members 7 are. brought together. The

bridge plug and easing are thus heated and indented and the material displaced merges together to form a continuous, non-porous weld between the plastic components of the detonator.

From the foregoing, description, it is apparent that many modifications of the process hereinbefore described will" present themselves to those skilled in the art without departing from the spirit and scope of the invention as set forth in the appended claims. 7

Having thus-described the invention, what is claimed is:

I. In the manufacture of electric blasting caps, the process comprising heating an assembled electric blasting cap having a thermoplastic bridge plug and a thermoplastic case to a temperature above. the melting points of said thermoplastic materials in the area of the external line of juncture of the bridge plug and case, formingia furrow in the heated portion of the external surface of the bridge plug and another furrow in the heated. portion of the external surface of the case, said furrows being parallel to and co-extensive with theexternal juncture of. the bridge plug and case, and intermixing the molten material's displaced by said furrows over the external juncture of the bridge plug and the case; p

2. In the manufacture of electric blasting caps, the process'comprising heating an assembled electric blasting cap having "a thermoplastic bridge plug and a thermoplastic case'to a temperature above the melting points' of said thermoplastic materials in the area of the external line of juncture of the bridge plug and case, and subject ing the heatedarea to a deforming pressure whereby a beading spanning the said external line of juncture is formed, said beading consisting of an intermixture of the thermoplastic materials from the bridge plug and easing.

3. In the manufacture of electric blasting caps, the process comprising providing 'an assembled electric blasting cap having a thermoplastic bridge plug and a thermoplastic-case, contacting said electric blasting cap along the line of external juncture of the bridge plug and case with a forming tool having a groove in the face thereof, said groove spanning the juncture, and increasing the pressure of the grooved surface of the toolon the blast-- ing cap thereby forming a thermoplastic beading spaning the line of juncture between the bridge plug and the case.

References Cited in the file of this patent UNITED. STATES PATENTS 2,232,634 Roberts et' al; Feb; 1 8', 1 941 2,265,359 De Neumann' Dec; 9', 1941 2,401,991 Walton et al- Tune 11, 1946 2,415,045 S'eavey Jan. 28, 1947 2,617,914 Keller Nov. 11, -2 2,722,155 Pauls Nov; 1, 1955 2,738,827 Roll Mar. 20, 1956 2,760,551 Downey et al Aug. 28, 1956' 2,802,421 Home et'al Aug. 13', 1957 FOREIGN PATENTS 652,054 Great Britain -....Ap1'.. -l 8, 195-1. 

